Project 1: Innate and Adaptive Immunity to Microbial Flagellins in IBD. We co-exist with an abundant microbiota that interacts with us and contributes to our health. Host-microblota interactions in the intestine are complex, and when disordered, can result in chronic inflammatory bowel disease (IBD). We have previously discovered a cluster of flagellins that serve as immunodominant antigens of the microbiota. These flagellins stimulate mucosal immune responses in normal hosts, but can also induce pathogenic T cells responses resulting in IBD. These flagellins provide a probe of both the normal mucosal immune response, as well as the abnormal response occurring in IBD. In the previous cycle we have generated a CBirl flagellin T cell receptor transgenic mouse, which was used to discover a T regulatory-lgA pathway maintaining homeostasis with the microbiota in normal mice. The overall hypothesis of Project 1 is that CD4 T cell effector subsets in the intestine maintain homeostasis by a number of different pathways that can compensate for one another, but that these pathways have limits beyond which intestinal inflammation results. We will use CBirl flagellin T cell receptor transgenic and novel cytokine reporter mice to address the following aims. Aim 1 will ask whether IL-23 regulates the intestinal Treg-lgA pathway maintaining homeostasis with the microbiota. We will further test the hypothesis that IL-23 deficiency increases Foxp3 and decreases RORyt expression by CD4 T cells in the intestine, that these changes are reflected in the numbers of Tregs vs. Thi 7 cells present in the lamina propria, and that the homeostatic Foxp3:RORYt balance can be altered by neutralization of IL-6. Aim 2 will test the hypothesis that mucosal Thi7 cells constitute a second pathway of homeostasis with the microbiota via IL-17 and IL-22 effects on the epithelium, and that this Th17 pathway is expanded in IgA deficient mice that are deficient in the Treg-lgA pathway. Aim 3 will determine the mechanisms of control of the intestinal Thi7 response to the microbiota focusing on IgA deficient mice. This Aim will determine whether perturbation of intestinal homeostasis in IgA deficient mice by impairment of CD4 Tregs or transfer of exogenous CBirl Thi 7 effector memory cells, results in colitis and whether these perturbations result in a shift in the Foxp3:RORYt balance. Lastly, this Aim will test whether adoptive transfer of exogenous Tregs, either IL-10-producing or FoxpS, to colitic B6.lgA-/- mice can restore regulation of mucosal Thi 7 responses and of the downstream elements regulated by Thi7 cytokines, despite active mucosal inflammation. These studies will provide important new understanding about mechanisms of CD4 T cell maintenance of homeostasis, as well as the limits of these homeostatic pathways. This project will interact extensively and is highly complementary to Projects 2, 3, and 4.